Make num_classes optional, in case of micro averaging

[baskrahmer]

What does this PR do?

Fixes #2837

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• Did you write any new necessary tests?

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📚 Documentation preview 📚: https://torchmetrics--2841.org.readthedocs.build/en/2841/

[Borda]

@baskrahmer how much is missing here?

[baskrahmer]

@baskrahmer how much is missing here?

I just need to check why the CI is failing.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 41%. Comparing base (276b90b) to head (db62975).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2841     +/-   ##
========================================
- Coverage      68%     41%    -27%     
========================================
  Files         332     332             
  Lines       18966   18970      +4     
========================================
- Hits        12955    7742   -5213     
- Misses       6011   11228   +5217     

[lantiga]

Thanks!

[lantiga]

Actually, shouldn't we check whether num_classes is not passed when microaveraging is not None and produce an informative error in that case? Otherwise we silently default to 1 and that wouldn't be desirable IMO.